I'm going to re-use the yeast cake from a low-gravity batch to ferment a high-gravity batch.

Can I just put the wort directly on it? Does it then need to be agitated?

Is it worth the risk of contamination to take the whole yeast cake out, wash it, strain it, and actually re-pitch it?

Is there a term for this complete re-use of a yeast cake?

UPDATE

Pouring down on the yeast cake of the previous batch is something I do regularly and with great effect. I don't remove anything from the carboy and I only step up in gravity. This might be helped by the fact that I only use leaf hops and they do a great job latching onto break material. I also use a boil screen, so there's very little trub anyway.

Yes, you can put the wort directly in. No, it doesn't need to be agitated, but a bit of shaking for aeration doesn't hurt.

I can't speak to risk of contamination with removing and repitching slurry, but I can say that the risk of contamination with pouring down is less.

FURTHER UPDATE

I still do this regularly with good results. I wanted to do some testing on how much ester production is inhibited by this degree of overpitching, so I tried making a light saison, with Wyeast French Saison (3711). (It's amazing yeast, by the way.) It was effectively Northern Brewer's Petite Saison d'Ete recipe, but at a 1.036 gravity. I made that batch, kegged it, then poured down a 1.080 biere de garde onto the cake. Fermentation was vigorous and healthy. The beer is missing nothing in the ester/character department. This beer had as much character and interest as anything I've made, even with being massively overpitched by homebrew standards.

Long story short: you can make very good beer like this, especially big beers.

For a better experiment though I'd rather try it with the same beer twice. Make an English Pale ale of 1050 OG with an estuary yeast. Pitch from a traditional starter for the first beer. Then rebrew the same recipe and use the whole cake. Then compare them side by side. I might try that actually.
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brewchezJun 11 '11 at 1:01

3 Answers
3

I used to pitch on cake but then started repitching less of the yeast. I'd say its better to grab a sanitize quart jar and pitch a small slug of the cake.
Aerate your wort as usual and pitch the slug of yeast.
This is the easiest way to reuse if you brew at the same time as racking something else over.

You'll get some cell growth which is important for proper flavor characteristics of the yeat you are using. Otherwise, you'll be missing out on some of the character the yeast can (and should) provide to the new beer.

Of course you can look into using the pitching counter at Mr.Malty. He talks about using slurry there too. But if you want to keep it easy, I suggest the scooping out a half-quart and going to it. Its only a minor increase in complexity over dumping cool wort on the entire cake and but I think the results are better than pitching on the whole cake.

Not to mention in the whole cake you also have alot of the protein and break material from the previous batch and the new batch. This can give you more headaches with lost wort/beer volume, sluggish ferments, more blow off, maybe even more time to clear and haze issues.

A friend of mine does this all the time. I've only done it once because I usually don't brew similar beers that close together. But now that my brew-partner and I are trying to perfect a single recipe, we're considering reusing brewing a new batch on bottling day, so we can re-use the yeast.

Just chill your wort and pour it right over the yeast, that should provide enough agitation. Stir if you need to.

I would add that you still need to aerate the wort. Angelo's comment on style similarity echos my own experiences. The best instance I have had of just pouring down on the cake was brewing a couple of Belgian golden. Started with a 5%ABV potential wort, let it ferment, and then pitched the 10%ABV potential triple down on the cake of the first beer. Made a dandy starter.
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TinCoyoteMar 25 '10 at 18:33

Why would you still need to aerate the wort? Aeration assists in yeast growth and you're likely close to over pitching when going on cake. Would you expect much growth at that cell density?
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brewchezMar 26 '10 at 13:30

Pro brewers pitch onto the yeast cake and still aerate. There may be something to it other than yeast growth.
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Dean BrundageMar 26 '10 at 14:07

2

The only reason I know of to aerate is to provide O2 to synthesize sterols for cell reproduction. Just because "pros" do it doesn't make it correct or mean that we need to.
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Denny Conn♦Jun 10 '11 at 18:01